A decanter type centrifuge generally includes a rotatable bowl having a coaxially mounted screw conveyer therein. The bowl is rotated at a constant but variable speed in order to create a centrifugal force to separate a fluid feed mixture into its constituent parts. The heavier portion of the feed, typically called solids because of its, at least partially, conveyable nature, collects on the inner surface of the bowl due to centrifugal force. The screw conveyor is rotated at a relative speed with respect to the bowl. This differential rotation creates a differential action between the flights of the screw and the bowl wall resulting in the conveyance cf the solids along the bowl wall. This differential speed can be varied during the operation of the centrifuge depending on certain parameters and the desired output qualities of the separated constituent parts of the feed mixture. The light or liquid portion of the feed moves radially inward of the heavier solids as a result of the centrifugal force. Thereafter, the separated heavy and light materials are separately discharged, typically from opposite ends of the bowl.
There are many structures and methods for creating a differential speed between the bowl and the conveyor of a decanter centrifuge. One type structure includes two electric motors. The first or main drive motor rotates the bowl. The second or backdrive motor rotates the conveyor at a second speed through a gear box. Because of the torques and friction created in rotating a liquid and solids filled bowl, the conveyor will want to rotate at the speed of the bowl. During operation, the backdrive motor usually acts as a brake, receiving torque from the conveyor. The backdrive motor (acting as a brake) maintains constant controlled speed, thus maintaining a constant differential speed between the bowl and the conveyor. During braking, the backdrive motor generates electrical power. Power is also generated by the backdrive motor when the speed of the conveyor is decreased. Furthermore, a decrease in speed of operation of either the backdrive and the main drive motors can result in the generation of power from either or both motors.
It is advantageous that motors utilized for a decanter centrifuge are variable in speed. Thus, the speed of rotation of the bowl and the conveyor may be controlled independently while operating. Both AC and DC type motors may be utilized as the main drive motor and/or the backdrive motor. However, DC type motors are generally more costly. Also, in certain applications, DC motors are not easily applicable. This is particularly true in hazardous situations where an explosion proof operation is required, due to the constant sparking that is involved within a DC motor operation. Within these explosion proof type operations there are certain levels of hazard. Division One, normally hazardous, requires an explosion proof AC or DC motor. Division Two explosion proof operation is classified as not normally hazardous. In this Division Two situation two faults would be required in order for an explosion to occur. A non-explosion proof AC motor or an explosion proof DC motor is typically required in this Division Two type situation.
The regeneration of power is often available within a centrifuge. Typically, a DC motor can be utilized to regenerate power when braking and to return that power back into the AC input line. However, DC motors are substantially more expensive than AC motors. Moreover, in explosion proof operations, the DC motor cost differential is further increased. In Division One/normally hazardous, an explosion proof AC motor would be cheaper than the DC explosion proof motors by a factor of 5. In Division Two (not normally hazardous), non-explosion proof AC motors are usually acceptable for proper operation.
Further advantages in reliability and ease of maintenance are obtained by using AC type motors. DC motors use commutators and brushes which wear and must be replaced often, especially in the corrosive environment usually seen by decanters. AC motors are simpler and use less parts.
Although power regeneration was possible for feeding back into the original AC power line, one problem that was not addressed was the regeneration of power directly from the backdrive motor into the main drive motor. Also, with the usual regenerative variable frequency control, the AC regenerated wave form distorts the normal form of the AC supply.